To define the maximum tolerated dose of phenylbutyrate administered as a continuous infusion in children with refractory cancer. To determine the dose-limiting toxicity and the incidence and severity of other toxicities of phenylbutyrate administered as a continuous infusion. To determine the pharmacokinetics of phenylbutyrate administered as a 4 gm/m2 iv infusion on day 1, cycle 1 of therapy. To determine the steady state plasma concentrations of phenylbutyrate and its metabolite, phenylacetate, and to attempt to correlate plasma concentrations of phenylbutyrate with clinical toxicity.